Cathodic protection is commonly used to control the corrosion of reinforcing steel, particularly in chloride contaminated concrete. Various types of impressed current cathodic protection anodes have been developed for reinforced concrete structures. The anode is one of the most critical components in such systems as it is used to distribute cathodic protection current to the reinforcing steel.
One of the most effective and durable anodes is made of a material which is resistance to corrosion, for example a mixed-metal-oxide (MMO) coated titanium substrate. MMO coated anodes are manufactured by coating a mixture of precious metal oxides on a specially treated precious metal. The coated substrate undergoes multiple thermal treatments at elevated temperatures to gain good bonding properties between the substrate and the coating. Although titanium is widely used as substrate material due to its resistance to corrosion, resistance to chemical attacks and high mechanical strength, other anodes such as tantalum, niobium and zirconium anodes are also used for different applications.
Since the first MMO-coated titanium anode was developed in 1984, many concrete structures have been protected using this material. To install the anodes, however, they must be embedded in concrete or cementitious grout. For example, titanium mesh with a concrete overlay, titanium ribbon or ribbon mesh embedded in cementitious grout in saw-cut slots, or discrete anodes embedded in grout in drilled holes. However, these types of the installation add some burden to the structure and some durability concerns. A useful review of MMO-coated anodes and installation techniques may be found in “Cathodic Protection of Steel in Concrete” By Paul Chess, Taylor & Francis (1998), ISBN 0419230106, the entire content of which is incorporated herein by reference.
Titanium-based anodes typically use iridium- or ruthenium-based precious mixed-metal-oxide (MMO) coatings. Iridium-based coatings are generally used for cathodic protection for reinforcing steel in concrete structures. When iridium-based anodes are operated in chloride-contaminated concrete at low current densities (i.e., less than 110 mA/m2 on the anode surface), oxygen gas is produced. However, if the anode is operated at a higher current density, the production of chlorine gas becomes the main anodic reaction.
If the chlorine gas accumulates at the anode-concrete interface, it becomes hypochloric acid. Once the acid concentration reaches a sufficiently high level, the cement past in the concrete matrix dissolves. The cement paste in the concrete is the electrolyte, which passes the ionic current from the anode to the reinforcing steel. However, if the cement paste is dissolved by the acid, only aggregates exist at the anode-concrete interface. Because most aggregates are not ionically conductive, the resistance of the anode-to-concrete increases significantly. As a result, the anode can no longer discharge the cathodic protection current. Consequently, the current density of iridium-based MMO coated anodes used in concrete is typically limited to 110 mA/m2 to prevent the acid generation.
U.S. Pat. No. 6,332,971 discloses a tubular anode made of titanium suboxide. A hole is formed in concrete that is larger in diameter than the anode, thereby creating a clearance. The clearance is filled with a gas-permeable material through which chlorine gases can be vented, purportedly allowing the anode to be operated with current densities as high as 1 A/m2. However, when anodes operate at the high voltages needed to produce such a high current density in concrete, the water in the concrete around the anode is dried out in a short period of time. As a result, the anode cannot maintain the high current density, and eventually diminishes due to the high resistance.
In some cases, a high-voltage power supply may be used to compensate for the problem of increasing electrical resistance. However, as voltage is increased, moisture at the anode-concrete interface is driven away at a faster rate due to the bipolar characteristics of water. This is the same principle of the electro-osmotic dewatering technique used to dry concrete. Therefore, by increasing the voltage, the resistance increases.
This situation may also be encountered with anodes used in a soil environment. When the impressed current anodes are operated in soil above the water table, the anode-to-soil resistance increases with time. If MMO-coated titanium anodes are used, a back voltage is created. The back voltage is defined as the potential of the MMO anode being more positive or noble as compared to the steel (tanks, pipelines, etc.) in the soil. When the power supply is connected to the anode and the steel to provide cathodic protection, the back voltage must be reversed before the anode discharge the current. This requires operating the anode at a higher voltage, causing the soil to dry faster. As a result, the anode-to-soil resistance increases and reaches the maximum voltage of the power supply. The cathodic protection current decreases and sometimes diminishes altogether. If this happens, the cathodic protection system is no longer effective, even if the anode is designed for a long life (i.e., over 40 years) and is still intact.
Another disadvantage of the anode described in U.S. Pat. No. 6,332,971 is that a bare titanium coil is used to feed the current to the titanium suboxide tubular anode. When the titanium coil is inserted into the tube, the titanium wire makes physical contact to the inside wall of the anode tube. However, since the titanium wire is not metallurgical connected to the anode tube directly, a high contact resistance exists between the wire and the tube anode.
For MMO anodes used in soil, including solid tubular or rod anodes, an electrical cable is used to connect to the anode. For the tubular anode, an expanded anchor which is pre-attached a copper cable is inserted inside the tube for the connection. If the anchor does not expand enough, a high contact resistance to the tubular wall is developed. The inside of the tubular anode is then filled with epoxy to seal the anode connector from water intrusion. However, if any moisture permeates into the connection, the copper cable discharges the cathodic protection current and corrodes in a short period of time. A further concern is physical damage. If the cable is over-stressed, the connection is broken, and once the connection is broken, it is not repairable. As a consequence, connection failures or the development of high resistance at the connection are often serious problems.